Underground gas storage process

ABSTRACT

961,415. Storing gases under pressure. GULF RESEARCH &amp; DEVELOPMENT CO. Sept. 18, 1961 [Sept. 30, 1960], No. 33300/61. Heading F4P Fuel gas is stored underground in waterbearing porous rock by injection through wells 12 after primary injection of cheap inert gas, e.g. nitrogen, scrubbed flue gas, or air which may contain oxygen consuming oil and may be followed by nitrogen, which inert gas is of a volume at least as large as that volume of fuel gas which would otherwise have been trapped upon withdrawal of the fuel gas. To detect contamination of the fuel gas by the inert gas, at least 20% of the inert gas is mixed prior to injection with a radioactive tracer gas, e.g. krypton 85, and a radioactivity detector in a chamber 23 is incorporated in a gas delivery pipe 22, the detector being electrically connected to an amplifier and counting rate meter 26 which, via a relay 27, controls electrical power from terminals 28 to a motor-controlled throttling valve 24 to shut-off or restrict gas delivery.

March 30, 1965 M. R. J. WYLLIE 3,175,614

UNDERGROUND GAS STORAGE PROCESS y O o o o y y y y fl O O O O M o o p o o/o e INVENTOR. MALCOLM R. J. WViL/E MW A TTORNE Y.

United States Patent O 3,175,614 UNDERGROUND GAS STORAGE PROCESS Malcolm R. J. Wyllie, Indiana Township, Allegheny County, Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Sept. 30, 1960, Ser. No. 59,540 11 Claims. (Cl. 166--42) This invention pertains to underground gas storage systems and their operation and in particular relates to an improved system for storing gas in an aquifer.

It has been found desirable and economical to store valuable gases, such as hydrocarbon gas, inunderground rock formations. Depleted gas fields have been extensively and successfully used for this purpose but unfortunately such gas fields are not always in geographically advantageous locations. Storage cavities in certain types of impervious rocks have been made by mining or leaching, and such cavities have been successfully used for gas storage, but they are very expensive. It has also been found advantageous to store gas in a water-hearing porous rock, commonly called an "aquifer. Such aquifers are commonly found in many localities. They serve as satisfactory underground reservoirs because in many instances they may possess all the structural features of a conventional hydrocarbon trap without however having any hydrocarbons naturally present.

It is of course necessary that the gas be injected at the top of a rock structure that will act as a gas trap since otherwise the injected gas will migrate away and be lost. The trap may have the configuration of any of the various known types of petroleum traps such as are produced by geological folding, change in dip, or faulting of subsurface formations, except of course that the trap is principally water saturated. Accordingly a conventional anticlinal or dome-shaped trap-forming structure of an aquifer is located and employed. Otherwise satisfactory aquifers having such structural traps are relatively abundant. A well is drilled into the aquifer at the structurally high point of the trap. Gas is then injected to and withdrawn from the aquifer through the well. If the hydrodynamic gradient technique is used to confine the gas, injection should be at the center of the trap thereby created.

When gas is injected into an aquifer, water in the pores of the rock is displaced, at a substantially constant pressure if in an open aquifer, or at a gradually ncreasing pressure in the case of a closed aquifer. It has been found that the water pressure in an aquifer depends on its depth and is usually about 0.43 p.s.i.g. per foot of depth (i.e. pressure of the water column) when the aquifer is open to the surface and about 0.7 p.s.i.g. per foot of depth when the aquifer is closed, and in no case exceeds 1.0 pound per square inch per foot of depth, this being the pressure of the rock column. The injection gas pressure must of course exceed the water pressure in the aquifer (reservoir pressure). The final reservoir pressure may not exceed that required to cause fracturing or lifting of the overburden. Accordingly a gas storage reservoir system can be operated at injection and withdrawal pressures between these two extremes. In instances involving a closed aquifer, it may be necessary aquifer, the water is pushed back and is displaced by gas at a pressure exceeding the reservoir pressure only by the small amount required to overcome capillary forces and to effect flow through the aquifer rock. However, when the gas pressure at the well head is subsequently reduced to permit the gas to flow back out of the rock, it is found that the water does not return completely to its former state of saturation in the aquifer rock. There remains residual irreducible gas saturation that is very substantial in amount depending on the nature of the aquifer rock. The aquifer does not return to perccnt water saturation and some of the initially injected gas remains permanently trapped.

In the succeeding cycles of gas injection less gas is of course required to fill the reservoir to the same extent as before, and a Volume equal to that of the newly injected gas can subsequently be withdrawn. However, it is never possible to withdraw a Volume of gas equal to that initially trapped in the rock pores, and any attempt to do so will merely result in withdrawal of large amounts of water. The trapped gas will be under a pressure equal to the hydrostatic reservoir pressure and it will be distributed throughout the region initially occupied by injected gas. The relative amount of the trapped gas depends on the conditions of the reservoir, i.e. rock type, permeability, porosity, etc., but for a common aquifer the trapped gas may represent a very substantial valuable investment which can never be recovered. This loss has served as a deterrent to gas storage in aquifers even though this type of reservoir is abundantly available.

It is an object of this invention to reduce the required initial investment in a gas storage reservoir in an aquifer.

It is a further object of this invention to provide a method of Operating an aquifer type gas storage reservoir in a more economical manner.

It is a further object of this invention to provide a method of Operating an aquifer type gas storage reservoir at a higher recovery efiiciency for stored gas than has heretofore been possible.

These and other important objects are attained by the invention described in this specification of which the drawing forms a part, and in which- FIGURE 1 is a top View of an example of a number of injection-production wells drilled into a virgin aquifer on the top of a dome-shaped structure,

FIGURE 2 is a cross-section of an example of a domeshaped aquifer structure employed in this invention, and

FIGURE 3 is a schematic diagram of an example of a well control system that may be employed in this invention.

In accordance with this invention the underground storage of a valuable gas in an aquifer having' the configuration of a gas trap is preceded by the injection into the virgin aquifer of a cheap inert gas at a pressure at least as high as the hydrostatic pressure in the aquifer at the location of storage, and of Volume at least as large as the Volume of gas which would normally be trapped as a residual gas saturation. The initially injected cheap gas is followed by the valuable gas injected for the purpose of storage and which is removed for use as required. Contrary to what one might expect it has been found that there is substantially no mixing of the cheap cushion gas with the valuable stored gas, and the principal commingling that occurs is the result of molecular ditfusion which is a very slow process. Accordingly only a very small fraction of the stored gas becomes contaminated with the cheap inert cushion gas. The small amount of contaminated gas represents a small degree of inefiiciency. Even over long storage times and many cycles of storage operation, the value of gas lost through contamination is in the aggregate much less than the value that would have been irrecoverably lost by the heretofore used method of injecting valuable gas from the very start of the storage project.

By way of example, consider that a virgin aquifer is available in the form of a water-filled sandstone 120 feet thick and having a dome-shaped gastrap structure whose top is at a depth of 1750 feet. It will be assumed to be known from geological studies that the structure is tight and that the aquifer is of satisfactory permeability so as to provide an acceptable underground storage reservoir. The virgin aquifer is of course 100 percent saturated with water, and laboratory tests on cores of the aquifer rock show that after gas is flowed through the core and water displaced, a residual gas saturaton of 36 percent of the pore Volume remains when the water is permitted to reimbibe and the injected gas is produced. According to the heretofore known method of operating a gas storage project in such an aquifer, if fuel gas were to be injected into the virgin aquifer and the gas subsequently withdrawn it would be found that only 64 percent of the fuel gas would come out, and any attempt to withdraw more gas would produce water. Accordngly 36 percent of valuable gas would be irrecoverably lost. Subsequently when gas is injected into the now gas-saturated part of the reservoir, all such subsequently injected gas can be substantially all recovered. Accordngly the second cycle, provided it did not further encroach on virgin parts of the aquifer, would be 100 percent efficient in that the entire amount of gas injected in the second cycle could be withdrawn and recovered. Succeeding cycles would be equally eflicient. However 36 percent of fuel gas initially injected into virgin reservoir formation would be permanently lost.

In the application of this invention to such a storage project there is first injected into the virgin aquifer an amount of cheap preferably inert gas, as for example nitrogen, fine gas that is substantially devoid of free oxygen, or the like. The initially injected gas (as well as the stored gas) should be such as to remain in the gaseous state under conditions of the reservoir temperature and pressure without condensation or liquefaction of any of its constituents. An amount of inert gas is injected into the virgin aquifer so that at the reservoir pressure the inert gas will occupy the formation pores in the storage region and thus satisfy the residual gas saturaton in the storage region.

The fuel gas to be stored is then injected. When the stored fuel gas is now taken out, it willbe found that substantially all of the stored gas can be recovered. If the time in storage is relatively short, it will be found that only an insignificent part of the stored gas is contaminated with inert gas. It is thus seen that by employing this invention the operator of the storage project is saved the expense of initially njecting and permanently losing a large quantity of valuable fuel gas. substantially the only offsetting expense is the cost of the inert gas first injected. The cost of such inert gas is likely to be only -20 percent of an equivalent Volume of fuel gas.

Referring to the drawing, in particular FIGURES 1 and 2, there is illustrated by way of example the structural reservoir previously mentioned. The aquifer 10 is water saturated and has a dome-shaped structure whose apex is for example 1750 feet below the earths surface 11. The high portion of the structure has an areal extent of 15,000 acres. A number, for example twenty or more, njection-production wells 12 are drilled and appropriately completed in the aquifer 10. The wells 12 may for example have a 660-foot or more spacing over the high portion of the reservoir in accordance with good engineering practice. The wells are equipped with appropriate valves 13 and are connected to appropriate manifolds, meters, pumps, etc. (not shown) in conventional manner. Initially there is injected into the virgin aquifer 10 through the wells 12 an amount of cheap available inert gas, as for example nitrogen or scrubbed ue gas, at a pressure re quired to overcome the existing reservoir pressure. Re-

ferring to the example, in order to satisfy the irreducible gas Volume of the virgin aquifer for a substantial storage region there is injected 1.8 billion standard cubic feet of inert gas into each well, i.e. a total of 36 billion standard cubic feet of inert gas is injected into the structurally high region of. the aquifer. The reservoir pressure after injection of the inert gas is 780 p.s.i.g. Following this 3.2 billion standard cubic feet of natural gas is injected into each well causing an increase in reservoir pressure to 1080 p.s.i.g. Since there are twenty wells, there is a total of 64 billion standard cubic feet of natural gas stored in the reservoir. The outer edge of the storage region is illustrated in FIGURE 1 as being roughly in the vicinity of the shaded Outline 14. The storage region is bounded by the gas-water contact illustrated by 15 in FIGURE 2. The gas storage facility functions in conventional manner with cyclic withdrawal and storing of natural gas within an operating reservoir pressure range of 780 to 1080 p.s.i.g. and aceompanied by only minute traces of the inert cushion gas. It is apparent that an initial capital saving has been effected in that 36 billion standard cubic feet of cheap inert gas was used to satisfy the irreducible gas saturaton instead of 36 billion standard cubic feet of expensive natural gas. 'Furthermore, from a conservation standpoint, no natural gas is lost in satiating the irrecoverable residual gas saturaton, i.e. substantially all of the injected natural gas is recoverable.

As stated above, there is an insignificant degree of commingling between the cushion gas and the stored gas. It is anticipa ted however that purchasers of the stored gas may wish to be assured that no contamination of the stored gas has occurred. Accordngly it is contemplated to provide in the process of this invention a monitoring and automatic shut-oil so that in the event that contamination appears in the withdrawn stored gas the well will automatically be partially or completely shut in. While contaminaton can be shown to be very minute, it is most likely to ocour -near the end of the withdrawal from storage. The advent of contaminated gas can easily be detect'ed by conventional chemical or combustion tests, but it is preferred to employ a radioaetive tracer technique. 'For this purpose the last injected portion, for example the last 20 percent of cushion gas injected into each well is intimately mixed prior to injection with a radioactive tracer gas in detectable concentration and whose half-life is long compared to the contemplated storage of the valuable gas. By way of example krypton having a half-life of 10.3 years may be employed as the nadioactive tracer. In the foregoing example, the last 20 percent of the inert or cushion gas injected into each well, namely the last 0.36 billion standard cubic feet of cushion gas injected into each well is mixed with 5 curies of krypton 85, so that the radioactive krypton is substantially uniform-ly distributed throughout this last part of the cushion gas. If commingling of the stored gas and the cushion gas occurs, the presence of the latter in the withdrawn gas can easily be detected by means of the radioact ive krypton mixed t-henewith. Accordingly after the stored gas has been injected and is to be withdrawn, each we'll is equipped with an automatic shut-oil as shown in FIGURE 3. The well 12 is equipped with conventional easing 20 and conventional tubing 21, the latter being in communication with the reservoir rock 10. The gas delivery pipe 22 leads from the tubing 21 .to a deteeting chamber 23, then to a motorcontrolled throttling valve 24, and to the delivery manifold (no-t shown). Inside the detecting chamber 23 is a conven- :tional radioactivity detector, as for example a Geiger counter, that is electrioally connected to a conventional amzplifier and counting-rate meter 26. The output of the amplifier 26 actuates the coil of a relay 27 which eontrols eleetric power from terminals 28 to a motor-controlled throttling valve 24. The circuits are arranged in wellknown manner so that upon the detection of radioactivity by the detector 25 the rate of gas flow from the well is reduced to such a value as to allow only .a predetermined very small degree of contamina tion to occur. Altematively, the control may be arranged to shut in the well completely upon detection of contamination.

The inert or cushon gas ha-s been described as being nitrogen, or flue gas that has been scrubbed to remove corrosive and condensible eonstituents, but this is by way of example only. It is contemplated that air may be employed as the cushon gas particularly in low-porosity reservoirs in which there is exceptionally little likel ihood of commingling of the cushon gas with the stored gas. It is further contemplated that more than one kind of cushon gas may be employed, as for example air, followed by substantially pure nitrogen. It is still further contemplated that when air is emp-loyed as cushon gas there may be mixed therewith at the time of injection an oxygen-consuming material, as for example crude oil or lins-eed oil, entrained in the air in the form of a fine mist. These materials are known to oxidize slowly and they will slowly consume i-n the reservoir the oxygen in the injected cushon air with 'which they are mixed. By injecting with the cushon air, when such is employed, the proper computed amount of these agents the oushion air is rendered, after a residence time in the format ion, substantially free f oxygen.

While the principal advantage of the process of this invention is the prevention of waste of valuable gases and the reduction o f capital expense in establishing a gas storage facility in an aquifer, an ancillary and important advantage results from the safety of initially injecting inert, i.e. non-combustible, gases so that any leaks that may exist in the reservoir may be found without hazard. Such leaks .may occur through old unrecorded or forgotten wells or through geological imperfections in the rocks overlying the aquifer. Such leaks may be located by conventional methods, as for example by making a radioactivity survey of the area to detect the radioactive tracer gas employed with the injected inert gas.

While the application of this invention has been described as applied to a natural dome-shaped structure, this is by way of example only and the invention is not intended to be restricted to natural trap reservoirs. The invention is also applicable to an artificially produced trap in a subsurface formaton having little or no relief. Such a flat reservoir is adapted for gas stonage by girding a selected area thereof with water injection wells to form a gascontaining hydrodynamic barrier. In using such artificial traps both the cheap inert cushon gas 'and the valuable gas being stored are injected near the center of such water-grded areas.

It is apparent that this invention is not applicable to cavity storage because in a cavity the convect-ion Currents will efect commingling of the inert gas with the valuable gas in a relatively short time. Hower, in an aquifer rock any injected gas occupies very fine pores and/or capillaries, and very little convection commingling takes place. A certain degree of molecular difiusion does of course occur but it is well known that the process of molecular diffusion is very slow, and it has been found to be so slow as to be insignificant during short periods of storage and even during long storage periods v ery little contami nation of the stored gas takes place.

It is contemplated that the initially injected inert gas may not immeditely occupy the entire Virgin reservoir region ultimately to be used for storage. An amount of inert gas required to satisfy the residual gas saturation of the entire storage region is initially injected and may be followed by stored gas and the storage facility operated, though possibly at less than ultimate Capacity and depending on permissible pressure limits. The stored gas will gradually push back the initially injected inert gas and the latter will eventually serve to occupy the Virgin rock pores ahead of stored gas, so that the inert gas satisfies the residual gas saturation in the Virgin rock pores over the entire storage region.

What I claim as my invention is:

1. In the underground storage of valuable gas in the pores of a subterranean aquifer having the conguration of a gas trap, the improvement which comprises injecting into the gas-trap region of the Virgin aquifer in situ a Volume of cheap inert gas different in composition from the gas to be stored sufi'icient to substantially saturate the region of said aquifer desired to be used for storage, said cheap inert gas being free from condensation at the aquifer pressure and temperature, and thereafter injecting into said gas-trap region of the aquifer the valuable gas to be stored.

2. In the underground storage of valuable gas in the pores of a subterranean aquifer having the configuration of a gas trap, the improvement which comprises injecting into the gas-trap region of the virgin aquifer in situ a Volume of cheap inert gas different in composition from the gas to be stored suflicient to substantially saturate the region of said aquifer desired to be used for storage at least to the extent of the residual gas saturation of the aquifer rock, said cheap inert gas being free from condensation at the aquifer pressure and temperature, and thereafter injecting into said gas-trap region of the aquifer the valuable gas to be stored.

3. In the underground storage of valuable gas in the pores of a subterranean aquifer penetrated by a well, the improvement which comprises injecting into the Virgin aquifer in situ a Volume of cheap inert gas different in composition from the gas to be stored through a well penetrating the aquifer, said Volume of said cheap inert gas being suflicient to saturate the region of said aquifer desired to be used for storage to the extent of the' residual gas saturation of the aquifer rock, said inert gas being free from condensation at the aquifer pressure and temperature, and snbsequently injecting valuable gas to be stored into the aquifer through the same well.

4. In the underground storage of valuable gas in the pores of a subterranean aquifer having the configuration of a gas trap and under a super-atmospheric reservoir pressure, the improvement which comprises injecting into the gas-trap region of the Virgin aquifer in situ at an injection pressure exceeding the reservoir pressure a Volume of cheap inert gas different in composition from the gas to be stored suicient to substantially saturate the region of said aquifer desired to be used for storage, said saturation being not less than the residual gas saturation of the aquifer rock, said cheap inert gas being free from condensation at the injection pressure and temperature, and thereafter injecting into said gas-trap region of the aquifer the valuable gas to be stored.

5. The process of claim 4 wherein said cheap inert gas is substantially free of oxygen.

6. The process of claim 4 wherein said cheap inert gas waea a theren p e aa cogsurningcompon t `"7.In the underground' storage of valuable gas in the pores of a subterranean aquifer having the configuration of a gas trap and under a super-atmospheric reservoir pressure, the improvement which comprises injecting into the gastrap region of the Virgin aquifer in situ at an injection pressure exceeding the reservoir pressure a Volume of cheap inert gas different in composition from the gas to be stored sufiicent to substantially saturate the region of said aquifer desired to be used for storage, said saturation being not less than the residual gas saturation of the aquifer rock, said cheap inert gas being free from condensation at the injection pressure and temperature, at least a last-injected portion of said cheap inert gas containing a detectable concentration of tracer material, and thereafter injecting into said gas-trap region of the aquifer the valuable gas to be stored.

8. In the underground storage of valuable gas in the pores of a subterranean aquifer having the configuration of a gas trap, the improvement which comprises injecting into the gas-trap region of the Virgin aquifer in situ a Volume of cheap inert gas different in composition from the gas to be stored suicient to substantially saturate the region of said aquifer desired to be used for storage at least to the extent of the residual gas saturation of the aquifer rock, said cheap inert gas being free from condensation at the aquifer pressure and temperature, the last-injected portion of said cheap inert gas containing a detectable concentration of tracer material, injecting into said gas-trap region of the aquifer valuablle gas to be stored, withdrawing gas from storage, and monitoring the withdrawn gas for the presence of said traceitrial. ""9'."The process of claim 8 wherein said tracer material comprises a radioactive Component whose half life is long compared to the storage time of the valuable gas.

10. In the underground storage of valuable gas in the pores of a subterranean aquifer penetrated by a well, the improvement which comprises injecting into the Virgin aquifer in situ through a well penetrating the aquifer a Volume of cheap inert gas having a composition different from that of the valuable gas to be stored, said volume being sufiicient to saturate the region of said aquifer desired to be used for storage at least to the extent of the residual gas saturation of the aquifer rock, said cheap inert gas being free from condensation at the aquifer pressure and temperature, the last-injected portion of said cheap inert gas containing a detegtabie concentratigngfi tracer matgial, thereafter injecting valuable gas to be stored i' the aquifer through said well, withdrawng valuable gas from storage through said well, mpnitgring the withdrgwnga QLhe presence of said tracer materid restricting gas withdrawal through said well M uporrdetecting said tracer material in the withdrawn gas.

11. The process of claim 10 wherein said tracer material comprises a radioactive Component whose half life is long compared to the storage time of the valuable gas.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Oil and Gas Journal, page 109, March 9, 1953. The Petroleum Engineer; Reference Annual, 1954; pp. E-21, E-22, and E-24. 

1. IN THE UNDERGROUND STORAGE OF VALUABLE GAS IN THE PORES OF A SUBTERRANEAN AQUIFER HAVING THE CONFIGURATION OF A GAS TRAP, THE IMPROVEMENT WHICH COMPRISES INJECTING INTO THE GAS-TRAP REGION OF THE VIRGIN AQUIFER IN SITU A VOLUME OF CHEAP INERT GAS DIFFERENT IN COMPOSITION FROM THE GAS TO BE STORED SUFFICIENT TO SUBSTANTIALLY SATURATE THE REGION OF SAID AQUIFER DESIRED TO BE USED FOR STORAGE, SAID CHEAP INERT GAS BEING FREE FROM CONDENSATION AT THE AQUIFER PRESSURE AND TEMPERATURE, AND THEREAFTER INJECTING INTO SAID GAS-TRAP REGION OF THE AQUIFER THE VALUABLE GAS TO BE STORED. 